1. Field of the Invention
The invention relates to an apparatus and to a method for identifying and localizing objects.
2. Description of Related Art
An identification of objects and goods which is as free of error as possible is necessary for the automation of logistical movements. This takes place at identification points, above all on a change of the owner of the good or on a change of the means of transportation or for a coordination of vehicle movements. An automating identification system is accordingly installed, for example, at an incoming goods area of a logistics center to record incoming and outgoing goods. This results in fast and traceable logistical movements. Further important applications for automatic identification are logistical distribution centers, for instance of package shippers, or the baggage check-in at airports.
RFID (radio frequency identification) technology allows RFID antennas (“transponders” or “tags”) also to be detected in an industrial environment. In this respect, in particular an unambiguous identification number can be read out. RFID transponders can basically be active, that is have their own energy supply and generate electromagnetic radiation independently. In practice, however, these transponders are less suitable for an industrial environment and logistics because the unit prices for such transponders cannot reach the low level required for the mass market due to the energy supply. Inexpensive passive transponders without their own energy supply are therefore usually used. In both cases, the transponder is excited to radiate the stored information by electromagnetic radiation of the reading device, with passive transponders taking the required energy from the transmission energy of the reading system.
In addition to the reading out of the information which the transponder bears, applications often also require the exact spatial localization of a plurality of transponders distributed over a small space. In the established ultra-high frequency standard ISO 18000-6, passive transponders are read out using the backscatter process. The detection region for an antenna is relatively large. It is possible to communicate within a plurality of cubic meters with the transponders located there due to the antenna characteristic with an opening angle of approximately 60° and a detection range of at least some meters. The reading unit accordingly provides a certain indirect statement on the presence location of the transponders. The scattering of the position, however, corresponds to the extent of the reading zone and thus amounts to a several meters. A more exact localization is thus initially not possible. A delineation of the detection region by special antenna characteristics or the detection range, for example via the transmission power, only solves the localization problem with restrictions, particularly since a cutting of the reading zone is often not wanted at all.
If it is then not guaranteed that only one object provided with a transponder is located in the relatively large reading zone, ambiguities arise. In this respect, the isolation, that is the association of an RFID reading with a specific transponder, is still solved by the protocol, at least in the ISO 18000-6 standard, in that only one respective specific transponder is prompted to transmit. However, in situations such as in the case of packages on a pallet in which a plurality of transponders are present, no spatial association can thus be made and it can thus not be determined where exactly transponders are located and which object they belong to.
If the objects provided with the transponders move on a conveyor belt in one direction, the localization is facilitated by this condition. It is namely possible at such conveying systems to assume a linear translation known via the sensed conveying system section and it is sufficient under these conditions to determine the position of the transponders once at a position which can be determined at a time which can be determined. A typical application is the association of a transponder with a specific object or package since the reading sequence does not necessarily also correspond with the order of the objects on the conveyor belt. A spatial resolution of only a few decimeters up to some meters is also achieved at conveyor systems due to the large detection region in the translation direction. A large number of error sources additionally interfere with the precision, for instance a variability due to the orientation of the transponder, field inhomogeneities and above all multiple scattering and echoes. The localization and association problem facilitated in this manner accordingly also still requires a solution.
There are approaches in the prior art to read out not only the information stored in the transponder using an RFID reader, but also to localize the transponder with the aid of the RFID signal. In this respect, the respective spacing of the transponder is estimated using different approaches such as the evaluation of the phase information, of the RSSI (received signal strength indicator) or the signal transit time. The localization then takes place by a plurality of measurements at different positions of the transponder or of the RFID reader or with the aid of spatially offset antennas. The localization of passive transponders is technically particularly complex and/or expensive inter alia due to their response behavior. A simple, exact localization is not achieved with these approaches. The spatial resolution, for example, still amounts to up to several meters due to the RSSI.
It is known from DE 199 404 03 A1 to use an optical sensor in addition to the RFID reader. The optical sensor can detect the position of transponders and an association then takes place by a comparison of optical information such as barcodes, the remission behavior or the outer geometry of objects with information stored in the transponder. This accordingly only works when corresponding optical information is separately stored in the transponder for its identification.
EP 2 012 253 B1 combines an RFID reader with a laser scanner. A plurality of fields are configured in the laser scanner which have to be passed through by an object in a certain order. An RFID reading is only then triggered or associated with the triggering object. This approach requires that objects to be identified only follow specific trajectories, in particular at a passage or an RFID gate. It is thus less a localization, since the location of the transponder is predefined it is not freely movable, but more a fixing of suitable reading times whenever a transponder is located at the predefined identification point. In addition, the spatial resolution of the laser scanner is relatively small due to the field-related detection. It is admittedly known per se also to localize objects more accurately using a laser scanner and to track its trajectory (object tracking). However, this would actually be counterproductive in EP 2 012 253 B1 since, unlike the rough trajectory detected by fields, which is mandated by the passing of the RFID reader, the precise trajectory of the objects would not be known at all in advance and would have to be abstracted only in a similar way as with fields to decide whether an object is passing the RFID in a manner provided for an RFID reading or not.
A phase based localization process for passive RFID transponders is known from the paper by R. Miesen et al., “Holographic Localization of Passive UHF RFID Transponders”, IEEE International Conference on RFID, 2011, p. 32-37. In this respect, a plurality of phase measurements of the RFID signal of a fixed-position transponder are carried out during the movement of the reading antenna along a known trajectory. A spatial probability distribution is superposed from this from which the transponder position is determined. An RFID reader moved on a known trajectory, however, often does not correspond to the demands on an industrial application where the RFID reader is stationary and the transponder is moved or, in the case of navigation using a moved RFID reader on a vehicle, the trajectory is actually not known. An association of a plurality of transponders with objects is not addressed in the paper.
DE 10 2009 030 076 A1 likewise deals with the localization of an object using a wave-based sensor based on residual phases with a holographic reconstruction. The antenna is here also moved on a known trajectory or on a trajectory estimated using accelerometers to generate a synthetic aperture.